Electron Transfer between the Quinones in the Photosynthetic Reaction Center and Its Coupling to Conformational Changes

Abstract

The electron transfer between the two quinones Q(A) and Q(B) in the bacterial photosynthetic reaction center (bRC) is coupled to a conformational rearrangement. Recently, the X-ray structures of the dark-adapted and light-exposed bRC from Rhodobacter sphaeroides were solved, and the conformational changes were characterized structurally. We computed the reaction free energy for the electron transfer from to Q(B) in the X-ray structures of the dark-adapted and light-exposed bRC from Rb. sphaeroides. The computation was done by applying an electrostatic model using the Poisson-Boltzmann equation and Monte Carlo sampling. We accounted for possible protonation changes of titratable groups upon electron transfer. According to our calculations, the reaction energy of the electron transfer from to Q(B) is +157 meV for the dark-adapted and -56 meV for the light-exposed X-ray structure; i.e., the electron transfer is energetically uphill for the dark-adapted structure and downhill for the light-exposed structure. A common interpretation of experimental results is that the electron transfer between and Q(B) is either gated or at least influenced by a conformational rearrangement: A conformation in which the electron transfer from to Q(B) is inactive, identified with the dark-adapted X-ray structure, changes into an electron-transfer active conformation, identified with the light-exposed X-ray structure. This interpretation agrees with our computational results if one assumes that the positive reaction energy for the dark-adapted X-ray structure effectively prevents the electron transfer. We found that the strongly coupled pair of titratable groups Glu-L212 and Asp-L213 binds about one proton in the dark-adapted X-ray structure, where the electron is mainly localized at Q(A), and about two protons in the light-exposed structure, where the electron is mainly localized at Q(B). This finding agrees with recent experimental and theoretical studies. We compare the present results for the bRC from Rb. sphaeroides to our recent studies on the bRC from Rhodopseudomonas viridis. We discuss possible mechanisms for the gated electron transfer from to Q(B) and relate them to theoretical and experimental results.

Time-resolved crystallographic studies of light-induced structural changes in the photosynthetic reaction center

“…Once Q B becomes a semiquinone, it is tightly bound in the proximal position, in agreement with the light structures from freeze-trapping studies (16)(17)(18), where it is stabilized by additional electrostatic or hydrogenbonding interactions due to the proton uptake by surrounding residues. Meanwhile, ubiquinone is still free to exchange with the solvent or intermediate site, and so the equilibrium shifts to complete proximal binding, as described by the pull-transition suggested previously (44). This equilibration is slower than the time delay in our experiment (3 ms), but shorter than the timescale of freezing in freeze-trapping studies (Ն150 ms).…”

“…We propose a model for the first electron transfer to Q B that accounts for the observation of a single binding site for ubiquinone in Fourier transform IR difference studies (41,42), the dependence of ubiquinone position on the protonation or position of ionizable residues within the Q B pocket, as suggested by simulations (22,23,(44)(45)(46), and the observed distal binding of ubiquinone in the dark (16)(17)(18)21). The distal binding site is a metastable binding site for neutral ubiquinone, but it does not correspond to a conformational gate in the reaction.…”

Energetics of Proton Transfer Pathways in Reaction Centers from Rhodobacter sphaeroides

“…In experiments, the mutation of Glu-H173 to Gln was found to slow down the first and second ET process from Q A to Q B , presumably by affecting the kinetics of PT to Q B , where Glu-H173 may participate (43). On the other hand, in steady-state FTIR measurements (44) and our previous computations (19,23), Glu-H173 in wild type bRC remains deprotonated regardless of the redox state of Q B . The latter fact implies a small pK a for Glu-H173, whereas at the same time, it does not exclude transient protonation that may be required for the PT events at Q B .…”

“…This result from kinetic measurements seems to be in conflict with steady-state FTIR measurements (44) and computational studies (19,23), since the latter two indicate an absence of proton uptake at Glu-H173 forming Q B Ϫ in the wild type bRC. However, the kinetic method focuses on transient states only as opposed to the latter methods that probe the steady state.…”

Key role of proline L209 in connecting the distant quinone pockets in the reaction center of Rhodobacter sphaeroides

“…The partial protonation events that occur on Q A Ϫ and Q B Ϫ formations have been studied by spectroscopic techniques by using site-directed mutagenesis (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) and by numerical methods (23)(24)(25)(26)(27)(28)(29)(30). There is a general agreement that the major response of the protein to the Q B Ϫ formation is the change of the ionization state of acidic residues situated in the Q B environment.…”